Re: next bio iters break discard?

[Kent Overstreet <kmo@daterainc.com>]

On Thu, Jan 16, 2014 at 12:21:10PM -0800, Hugh Dickins wrote:
> For me this just shifts the crash,
> from __blk_recalc_rq_segments() to blk_rq_map_sg():
> 
> blk_rq_map_sg
> scsi_init_sgtable
> scsi_init_io
> scsi_setup_blk_pc_cmnd
> sd_prep_fn
> blk_peek_request
> scsi_request_fn
> __blk_run_queue
> blk_run_queue
> scsi_run_queue
> scsi_next_command
> scsi_io_completion
> scsi_finish_command
> scsi_softirq_done
> blk_done_softirq
> __do_softirq
> irq_exit
> do_IRQ
> common_interrupt
> <EOI>
> cpuidle_idle_call
> arch_cpu_idle
> cpu_startup_entry
> start_secondary
> 
> It's GPF'ing on struct scatter_list *sg 0x800000001473e064 in
> 
> static inline void sg_assign_page(struct scatterlist *sg, struct page *page)
> {
> 	unsigned long page_link = sg->page_link & 0x3;
> 
> It appears to be in the static inline __blk_segment_map_sg(),
> and that GPF'ing address is what it just got from sg_next().
> 
> Sorry, this isn't the kind of dump you'll be used to, but it's the
> best I can do at the moment, and I've just had to reboot the machine.
> 
> O, tried again and it hit the BUG_ON(count > sdb->table.nents)
> on line 1048 of drivers/scsi/scsi_lib.c:
> 
> scsi_init_sgtable
> <IRQ> scsi_init_io
> scsi_setup_blk_pc_cmnd
> sd_setup_discard_cmnd
> sd_prep_fn
> blk_peek_request
> etc. as before

Ok, I reread the code and figured it out - the analagous change also has to be
made in __blk_segment_map_sg(). I'll mail out a patch for this tomorrow after
I've stared at the code more and had less beer.